A typical hard disk drive includes a series of magnetic disks or platens, each associated with a magnetic read/write head. The head, commonly known as a thin film head (TFH), comprises a reader and writer, is typically a monolithic device embodied within a slider. The head is formed analogous to an integrated circuit and includes magnetic elements forming magnetic write poles, a coil for generating a magnetic field for writing the disk, and magnetic sensor for reading the disk. The magnetic head is incorporated into a trailing edge of the slider. One face of the slider, known as the air bearing surface (ABS), is manufactured so as to ride on an air cushion very close to but above the hard disk surface. The ABS is polished by a series of grinding and lapping steps, to an atomic scale smoothness and planarity, so that it can be held in constant and very close proximity to the spinning surface of the hard disk. The ABS is contoured and includes etched features and cavities that enable the slider to ‘fly’ at a controlled and repeatable distance over the hard disk. This distance is termed the fly-height. The slider is suspended on an arm extending from a gymbal assembly in the drive, and the whole assembly is termed the head gymbal assembly (HGA). The side of the slider opposite to the ABS, known as the back side, is mounted to the arm. The spacing and position of the slider and the disk surface must be controlled to a tight tolerance to maintain the slider at a constant fly height, which is critical for accurate reading and writing of magnetic domains on the disk. Furthermore, the planarity of the head's ABS and the parallel relationship of the ABS to the head back side (the side mounted to the slider) must be tightly controlled so that the head flies above the spinning disk at the desired height across the entire ABS.
Hard disk technology continues to evolve to provide increasingly greater areal density, most recently with the transition from longitudinal magnetic recording (LMR) in which the written bit is in the plane of the disk to perpendicular magnetic recording (PMR) in which the written bit is perpendicular to the plane of the disk since the latter has greater potential density due to larger material volume per stored bit. Increasing density, however, enhances the criticality of device dimensions, and requires scaling down all dimensions associated with the head and slider.
For the manufacturing of hard disk drives of increasing areal density to be economic, variations in tolerances of the manufactured parts must be tightly controlled. One particular source of such variation is the thickness of the slider from the ABS to the back side, as variation in this thickness translates directly to variation in fly height. Low cost manufacture thus requires the thickness of slider from ABS to back to be tightly controlled.
Heads are typically fabricated in arrays on a wafer, in a grid pattern. The finished wafer is then sliced into wafer sections which are square or rectangular, and those sections are then sliced to produce rows of heads, or stacks of rows known as “rowstacks” which are subsequently sliced into individual rows. The rows are then lapped to achieve key reader and writer parameters, pattered to define the ABS topography, and encapsulated with a passivation coating before being diced to create individual sliders which may be mounted to drive mechanisms.
The shape of each slider is a function of the straightness of the row and die cuts that formed the slider, the perpendicularity and parallelism of the cut faces to each other, and the smoothness of the faces. Shape control is important because it not only sets the dimensions of the slider but also provides well-defined reference surfaces for subsequent operations such as lapping. Optical alignment is typically performed prior to each sawing/slicing step to ensure a straight cut. This involves aligning the position of the saw blade and its direction of motion relative to alignment marks (also known as fiducials) on the wafer. In addition to this initial alignment, typically sawing or slicing is feedback controlled, to ensure best alignment across a wafer section or rowstack. After sawing, the exposed surface of the wafer, which forms either the front-side or back-side of the next row (or rowstack), is ground to remove saw marks and achieve smoothness, typically using a fixed abrasive grinding wheel. Typically it is preferable to grind the back-side of the rowbar prior to slicing, rather than grinding the ABS prior to slicing, since the former establishes the principal reference surface for the head. The separated row or rowstack is then subjected to a sequence of steps to fabricate the individual sliders.
The lapping step noted above uses a lapping plate of a soft material, typically a Tin alloy such as Tin-Bismuth or Tin-Antimony. The lapping plate is typically a disc of, e.g., 16″ diameter, with a hole in the middle, e.g., 4″ in diameter.
The lapping plate is typically textured, such as by “soda blasting” i.e. sandblasting with baking soda, or by turning grooves into the plate using a diamond stylus.
Once plate is textured, it may be charged with abrasive from a slurry. One exemplary slurry is ethylene glycol and water containing diamond chips. Another exemplary slurry comprises an oil base with diamond chips. Depending upon the grit desired, the diamond chips typically range from 75-100 nm as the smallest size up to 1 micron as the largest size, although there is a distribution of sizes for any chosen grit. The size and morphology of the diamonds are selected based on the nature of the application.
The lapping plate is charged with diamond using a charging plate, typically a ceramic ring. The lapping plate is rotated against the ceramic ring under pressure, typically between 5-50 psi, with the diamond containing slurry between. After approximately 30-60 minutes of such rotation, the diamond chips embed in the lapping plate (and some minor abrasion of the lapping plate). The plate is then “charged” with the diamond abrasive.
Lapping plates are qualified by using an optical method to measure roughness of a specimen lapped with the plate.
The lapping process is similar to the charging process described above, but in lapping the rowbar is mounted to a row tool, typically a metal bar, which is itself mounted to a head that fine controls the position of the row tool and bar. The head then gently pushes the rowbar against the rotating lapping plate. The long axis of the rowbar, which is about 50 mm long, is typically placed in a radial direction relative to the lapping plate, and then the head supporting the rowbar sweeps from this position about an axis outside of the lapping plate, so that the rowbar moves across the lapping plate during lapping. This process helps to average the effects of grit irregularity or imperfections in the lapping plate.
In a typical lapping process for a magnetic head, the lapping plate and slurry are chosen in sequential steps of lapping, which will be known as Rough lap, Fine lap, and Kiss lap. Rough lapping uses relatively large diamonds in a slurry, and brings the slider to within one micron of straight across the ABS. Fine lapping involves relatively small diamonds in the slurry, and brings the slider to as close to straight as possible. Kiss lapping typically does not utilize any diamond in the slurry, but relies upon diamond embedded in the lapping plate only, to generate the desired surface smoothness.
A first critical dimension to be controlled in lapping is the “stripe height”, which is the height of the top edge of the sensor embedded within the head. The sensor is a stack of magnetically permeable materials adjacent to a magnetized layer. The layers are stacked on edge when the slider is flying above the disk. The degree of rotation of field in the sensor's free layer depends on the amount of material in the stack—too little mass, and the magnetization in the stack saturates, too much mass, and the magnetization will not change much. Thus, the size of the PMR sensor layers, controlled by the “stripe height” is a critical dimension.
A second critical dimension to be controlled in lapping is the “throat height” or “breakpoint height”, which is the distance above the air-bearing surface at which the magnetic pole tip embedded within the slider, widens from its narrowest width at the ABS as it extends from the ABS to the magnetic coil embedded within the slider. Throat height affects the concentration of magnetic field lines emerging from the pole and is optimized for best magnetic writing of domains on the disk.
To monitor and control the depth of lapping and thus the stripe height and throat height, heads typically include an electrically resistive element that extends between two external contacts on the head. By measuring the resistance between these contacts, the lapping tool may measure the amount of material that has been lapped from each head and thus control lapping. In the case of four contacts, the lapping rate for the writer and reader may be independently monitored.
While this electrical lapping guide method is useful in measuring the manner in which lapping is proceeding, there are numerous difficulties in lapping. A first difficulty is that rowbars are not perfectly flat, so even with electrically controlled lapping the lapping amount may not be controlled consistently on all sliders in a rowbar. Very low yield results from a nonflat rowbar, as nearly all of the sliders are lapped too little or too much to achieve the critical dimensions for throat height and stripe height.
One possible solution to this problem, is to provide a flexible row tool, such as of stainless steel, and provide the lapping tool's head with a means for bending the row tool (and thus the rowbar mounted to it) as lapping proceeds. By controllably bending the row tool and rowbar, the depth of lap can be at least partially equilibrated even if the rowbar is not perfectly flat.
Even with these developments, however, consistent lapping of rowbars has not been achieved. A first source of difficulty is that known lapping tools do not control bending of the row tool tightly enough to control lapping depth across an entire row bar. A second difficulty arises from the use of a flexible row tool. A flexible tool, made of material such as stainless steel, typically has a coefficient of thermal expansion that mismatches that of the rowbar. As a consequence, under temperature change (such as occurs during lapping, or during rowbar bonding which involves thermal cycling of a thermoplastic adhesive), the bar and row tool are thermally stressed as they differently expand or contract. The resulting bending of the bar and row tool exacerbates the problems discussed above. Unfortunately, a row tool that has a similar coefficient of thermal expansion as a rowbar, e.g. one made of ceramic material, is relatively stiff and is impractical for use with known lapping tools because known tools are unable to generate sufficient forces to bend a stiff row tool of this kind. A second source of difficulty is that very low and controlled lapping pressures are required to remove material controllably at an atomic (nm) scale. A third difficulty is that the angular orientation between the surface being lapped and the lapping plate must be precisely set before lapping is started to avoid the formation of facets and ensure that the read sensor and writer are lapped at similar rates.
Thus, there remain difficulties in lapping of magnetic storage heads that limit the ability to reliable create high density storage devices using known technology.